This invention applies in particular to the separation of air by cryogenic distillation. Over the years numerous efforts have been devoted to the improvement of this production technique to lower the oxygen cost which consists mainly of the power consumption and the equipment cost.
It has been known that an elevated pressure distillation system is advantageous for cost reduction and when the pressurized nitrogen can be utilized the power consumption of the system is also very competitive. It is useful to note that an elevated pressure system is characterized by the fact that the pressure of the lower pressure column being above 2 bar absolute. The conventional or low pressure process meanwhile has its lower pressure column operates at slightly above atmospheric pressure.
The higher the pressure of the lower pressure column, the higher is the air pressure feeding the high pressure column and the more compact is the equipment for both warm and cold portions of the plant resulting in significant cost reduction. However, the higher the pressure, the more difficult is the distillation process since the volatilities of the components present in the air (oxygen, argon, nitrogen etc) become closer to each other such that it would be more power intensive to perform the separation by distillation. Therefore the elevated pressure process is well suited for the production of low purity oxygen ( less than 98% purity) wherein the separation is performed between the easier oxygen-nitrogen key components instead of the much more difficult oxygen-argon key components. The volatility of oxygen and argon is so close such that even at atmospheric pressure it would require high number of distillation stages and high reboil and reflux rates to conduct such separation. The elevated pressure process in the current configuration of today""s state-of-the-art process cycles is not suitable nor economical for high purity oxygen production ( greater than 98% purity). Since the main impurity in oxygen is argon, the low purity oxygen production implies no argon production since over 50% of argon contained in the feed air is lost in oxygen and nitrogen products.
Therefore it is advantageous to come up with an elevated pressure process capable of high purity oxygen production and also in certain cases argon production.
The new invention described below utilizes the basic triple-column process developed for the production of low purity oxygen and adds an argon column to further separate the low purity oxygen into higher purity oxygen along with the argon by-product. By adding the argon column one can produce high purity oxygen (typically in the 99.5% purity by volume) required for many industrial gas applications and at the same time produce argon which is a valuable product of air separation plants.
The elevated pressure double-column process is described in U.S. Pat. No. 5,224,045.
The triple-column process is described in U.S. Pat. No. 5,231,837 and also in the following publications:
U.S. Pat. Nos. 5,257,504, 5,438,835, 5,341,646, EP 636845A1, EP 684438A1, U.S. Pat. Nos 5,513,497, 5,692,395, 5,682,764, 5,678,426, 5,666,823, 5,675,977, 5,868,007, EP833118.
U.S. Pat. No. 5,245,832 discloses a process wherein a double-column system at elevated pressure is used in conjunction with a third column to produce oxygen, nitrogen and argon. In order to perform the distillation at elevated pressure a nitrogen heat pump cycle is used to provide the needed reboil and reflux for the system. In addition to the power required for the separation of argon and oxygen in the third column the heat pump cycle must also provide sufficient reflux and reboil for the second column as well such that the resulting recycle flow and power consumption would be high.
U.S. Pat. No. 5,331,818 discloses a triple column process at elevated pressure wherein the lower pressure columns are arranged in cascade and receive liquid nitrogen reflux at the top. The second column exchanges heat at the bottom with the top of the high pressure column. The third column exchanges heat at the bottom with the top of the second column. This process allows to optimize the cycle efficiency in function of the ratio of low pressure to high pressure nitrogen produced.
None of the above processes can be used economically and efficiently to produce high purity oxygen or argon.
U.S. Pat. No. 4,433,989 discloses an air separation unit using a high pressure column, an intermediate pressure column and a low pressure column, the bottom reboilers of the low and intermediate pressure columns being heated by gas from the high pressure column. Gas from the low pressure column feeds an argon column whose top condenser is cooled using liquid from the bottom of the intermediate pressure column. In this case the intermediate pressure column has no top condenser and all the nitrogen from that column is expanded to produce refrigeration.
U.S. Pat. No. 5,868,007 discloses a triple column system using an argon column operating at approximately the same pressure as the low pressure column. Gas from the bottom of the argon column is used to reboil the intermediate pressure column.
According to the invention, there is provided a process for separating air by cryogenic distillation comprising the steps of
feeding compressed, cooled and purified air to a high pressure column where it is separated into a first nitrogen enriched stream at the top and a first oxygen enriched stream at the bottom,
feeding at least a portion of the first oxygen enriched stream to an intermediate pressure column to yield a second nitrogen enriched stream at the top and a second oxygen enriched stream at the bottom, sending at least a portion of the second nitrogen enriched stream to a low pressure column or to a top condenser of the argon column,
separating a third oxygen enriched stream at the bottom and a third nitrogen enriched stream at the top of the low pressure column, sending at least a portion of the second oxygen enriched stream to a low pressure column
sending a heating gas to a bottom reboiler of the low pressure column,
removing at least a portion of the third oxygen enriched stream at a removal point,
removing a first argon enriched stream containing between 3 and 12% argon from the low pressure column,
sending the first argon enriched stream to an argon column having a top condenser and a bottom reboiler heated by a gas stream, recovering a second argon enriched stream, richer in argon than the first argon enriched stream, at the top of the argon column and removing a fourth oxygen enriched stream at the bottom of the argon column.
It is useful to note that when a stream is defined as a feed to a column, its feed point location, if not specified, can be anywhere in the mass transfer and heat transfer zones of this column wherever there is direct contact between this stream and an internal fluid stream of the column. The bottom reboiler or top condenser are therefore considered as part of the column. As an example, a liquid feed to a bottom reboiler of the column is considered as a feed to this column.
According to further optional aspects of the invention:
that gas stream heating the bottom reboiler contains at least 90% nitrogen,
the gas stream heating the bottom reboiler of the argon column is at least a portion of one of the first, second and third nitrogen enriched streams,
the process comprises compressing at least a portion of the nitrogen enriched gas stream and sending it as heating gas to the bottom reboiler of the argon column,
the process comprises sending the fourth oxygen enriched stream to the low pressure column,
the argon enriched liquid is removed from the low pressure column in liquid form and sent to the argon column with a maximum gaseous content of 2%,
the process comprises removing the first argon enriched stream at least 20 theoretical trays below the point of maximum argon concentration in the low pressure column,
the process comprises removing the first argon enriched stream at most 30 theoretical trays below the point of maximum argon concentration in the low pressure column,
the process comprises removing the first argon enriched stream at the bottom of the low pressure column,
the process comprises removing the third oxygen enriched stream and the second argon enriched stream as products,
the third oxygen enriched stream contains at least 95% oxygen and the second argon enriched stream contains at least 95% argon,
the process comprises removing the first argon enriched stream at most 5 theoretical trays above the bottom of the low pressure column and removing the fourth oxygen enriched stream as a product,
the fourth oxygen enriched stream contains at least 95% oxygen,
the process comprises sending nitrogen enriched liquid from the top of the low pressure column to the top condenser of the argon column,
the heating gas for the bottom reboiler of the low pressure column is nitrogen enriched gas from the high pressure column or air,
oxygen enriched streams of differing purities are removed from the low pressure column,
the low pressure column operates at above 2 bar, preferably above 3 bar and most preferably above 4 bar,
oxygen enriched streams of different purities are removed from the low pressure column,
the argon column operates at a pressure at least 0.5 bar lower than the pressure of the low pressure column,
the intermediate pressure column has a bottom reboiler.
the process comprises sending a nitrogen enriched gas from the high pressure column to the bottom reboiler,
the process comprises at least partially vaporizing or subcooling at least part of the second nitrogen enriched fluid before sending it to the low pressure column,
the process comprises at least partially vaporizing or subcooling at least part of the second oxygen enriched fluid before sending it to the low pressure column,
the intermediate pressure column has a top condenser and the process comprises sending at least part of the second oxygen enriched fluid to this top condenser,
air is sent to the intermediate pressure column.
According to a further aspect of the invention, there is provided an apparatus for separating air by cryogenic distillation comprising a high pressure column, an intermediate pressure column, a low pressure column having a bottom reboiler and an argon column having a top condenser and a bottom reboiler, a conduit for sending air to the high pressure column, a conduit for sending at least part of a first oxygen enriched liquid from the high pressure column to the intermediate pressure column, a conduit for sending a second oxygen enriched fluid from the bottom of the intermediate pressure column to the low pressure column, a conduit for sending a second nitrogen enriched fluid from the top of the intermediate pressure column to the low pressure column or to a top condenser of the argon column, a conduit for sending a heating gas to the bottom reboiler of the low pressure column, a conduit for removing a third oxygen enriched fluid from the low pressure column, a conduit for sending a nitrogen enriched liquid from the high pressure column to the low pressure column, a conduit for sending a first argon enriched stream from the low pressure column to the argon column, a conduit for withdrawing a second argon enriched stream containing at least 50% argon from the argon column and a conduit for withdrawing a fourth oxygen enriched stream from the argon column.
According to further options:
the argon column has a bottom reboiler,
there is a conduit for sending a third nitrogen enriched stream from the low pressure column to the bottom reboiler of the argon column,
there is a compressor for compressing the third nitrogen enriched stream before sending it to the bottom reboiler of the argon column,
there is a conduit for sending a nitrogen enriched liquid from the top of the low pressure column to the top condenser of the argon column,
the conduit for removing the first argon enriched stream is connected to the bottom of the low pressure column,
there is a conduit for sending the fourth oxygen enriched stream to an intermediate point of the low pressure column,
there are means for pressurizing at least one oxygen enriched liquid withdrawn from the argon column or the low pressure column,
there are conduits for withdrawing oxygen enriched streams of differing purities from the low pressure column,
the conduit for removing the first argon enriched stream is connected to an intermediate level of the low pressure column,
there are means for at least partially vaporizing or subcooling the second nitrogen enriched liquid before sending it to the low pressure column,
there are means for at least partially vaporizing or subcooling the second oxygen enriched liquid before sending it to the low pressure column,
the intermediate pressure column has a bottom reboiler,
there are means for sending a nitrogen enriched gas from the high pressure column to the bottom reboiler of the intermediate pressure column,
the intermediate pressure column has a top condenser,
there are means for sending at least part of the second oxygen enriched fluid to the top condenser of the intermediate pressure column,
there are means for sending air to the intermediate pressure column,
there are means for expanding the first argon enriched stream sent from the low pressure column to the argon column, preferably constituted by a valve.
The new invention addresses this aspect by adding a argon column operated at relatively lower pressure to the elevated pressure triple-column column process to perform an efficient separation of argon and oxygen which is a necessity for the production of high purity oxygen and/or argon production.
In one embodiment (FIG. 1) the process can be described as follows:
Air free of impurities such as moisture and CO2 is fed to a high pressure column where it is separated into a nitrogen rich stream at the top and an oxygen rich stream at the bottom.
Feed at least a portion of the oxygen rich stream to a side column to yield a second nitrogen rich stream at the top and a second oxygen rich stream at the bottom. This side column has a reboiler which exchanges heat with the nitrogen rich gas at or near the top of the high pressure column. Recover a portion of the second nitrogen rich stream as liquid reflux and feed it to the low pressure column.
At least partially vaporizing at least a portion of the second oxygen rich stream in the overhead condenser of the side column and feed this vaporized stream and the non-vaporized portion to the low pressure column.
The low pressure column separates its feeds into a third oxygen rich stream at the bottom and a third nitrogen rich stream at the top. The bottom of the low pressure column exchanges heat with the top of the high pressure column. Recover at least a portion of the 3rd oxygen rich stream as oxygen product.
Extract an oxygen-argon stream above the 3rd oxygen rich stream. Feed this oxygen-argon stream to the argon column. Recover a argon stream at the top of the argon column and a 4th oxygen rich stream at the bottom of the argon column.